The instant invention relates to grooved press belts for use in paper making machinery, and other like machinery, and more particularly to an improved groove configuration for such press belts.
Press belts are used in various press devices, such as shoe-type presses, as used in paper making machines and calendars, to transport a continuous sheet through a press nip. The prior art press belts typically comprise a polyurethane or rubber material having a reinforcing fiber weave imbedded therein. In the paper making art, it is well known to provide grooves in the outer surface of a press belt in order to provide a channel to carry water away from the sheet as it is being pressed. In this connection, grooves are usually formed with a rectangular cross-section. However, there are two significant problems with conventional rectangular groove configurations. The first problem is that rectangular shaped grooves have a natural tendency to close under pressure of the nip. The pressure of the nip deforms the lands between the grooves forcing the sides of the grooves toward each other and thereby closing the groove. Several solutions have been proposed for the problem of groove closure. However, none of the proposed solutions appear to be satisfactory. U.S. Pat. No. 4,880,501 discloses a groove configuration wherein the tops of the lands are formed with a concave shape. This concave formation is believed to reduce the deformation of the land. However, this type of machining is very difficult to accomplish, and furthermore it does not completely eliminate groove closure. U.S. Pat. No. 4,908,103 discloses a press belt having an outer surface which is constructed of a harder material which is not as likely to be elastically deformed. However, the use of two different elastomers leads to problems with delamination or separation of the two elastomer layers. British Patent No. GB 8818992.3 discloses a press belt wherein cross-pieces extend between the lands to provide to support the lands. However, reinforcing cross-pieces reduce the efficiency of the grooves by hindering the flow of water once in the grooves. Furthermore, the cross-pieces are difficult to machine.
The second problem associated with conventional rectangular groove configurations is the tendency for the lands to break off at their base. Cracking of the belt leads to shortened life span, increased belt replacement, and increased machine down time. In this regard, it has been found that the cumulative stress of repeatedly passing through the nip causes the lands to crack at their bottom edges. The sharp corners of the rectangular grooves create stress points in the material wherein the maximum stress often exceeds four times the stress elsewhere in the material. In order to remedy this problem, it is has been suggested to make the grooved surface of the belt from a harder elastomer. However, there is the problem of delamination, as indicated earlier. In addition, harder elastomers generally have less tolerance for repeated bending as would be required in an press belt. One solution which has been suggested is to provide the elastomer with reinforcing threads (U.S. Pat. No. 4,946,731). However, when a plurality of filaments are used to reinforce the elastomer, cutting of the grooves exposes the matrix of fibers and opens paths for water to get inside the belt and cause failure. Yet another solution is to provide filaments located within the lands (GB 8818992.3). However, precise location of the filaments and machining of the grooves so that the filaments lie within the lands is extremely difficult, leading to a high percentage of substandard belts or belt rejections.
Accordingly, among the objects of the instant invention are: the provision of a groove configuration for a press belt which effectively reduces groove closure; the provision of a groove configuration which reduces cracking of the belt; and the provision of a groove configuration which is simple and inexpensive to machine.
The above objects are accomplished by providing a groove configuration wherein the grooves are formed with an arcuate bottom and two upwardly diverging side walls. The arcuate bottom is preferably semi-circular and has a diameter which is equal to about one half of the width of the groove opening. The upwardly diverging side walls preferably include radiused upper edges which provide a smooth curved transition between the side walls and the outer surface of the press belt. Each side wall preferably has an angle of divergence between about five (5) degrees and about fifteen (15) degrees from a vertical plane. In a second embodiment, the groove is formed with a substantially flat bottom and radiused corners which provide a smooth transition between the flat bottom and the upwardly diverging side walls.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.